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Notes: The body-fixed Hamiltonian for a trimer of polyatomic monomers is expressed in terms of the distances between the monomer centers of mass and of the orientation angles of the monomers relative to the body-fixed frame. The particular case where the monomers are water molecules is considered in some detail. In this work we attempt to assess the possibility of using a computer algebra system to aid in the evaluation of the angular matrix elements. © 1995 American Institute of Physics.

Notes: The inclusion of the fragment relaxation energy terms in the estimation of the basis set superposition error (BSSE) correction to the interaction energy is necessary in order to ensure formal convergence to the uncorrected result at the complete basis set (CBS) limit. The problems associated with their omission are demonstrated for F−(H2O), Cl−(H2O), and (H2O)2 especially when very large basis sets are used. The family of correlation consistent basis sets allows for a heuristic extrapolation of both uncorrected and BSSE-corrected electronic energy differences of the three complexes to the MP2 CBS limits of −27.1, −15.1, and −4.9 kcal/mol respectively. © 1996 American Institute of Physics.

Notes: The optimal structures and harmonic vibrational frequencies of ring water clusters (H2O)n, n=1–6 are computed using density functional theory (DFT). The exchange functionals of Slater (S), Becke (B), the correlation functionals of Lee–Yang–Parr (LYP), Vosko–Wilk–Nusair (VWN), Perdew's local (PL), and gradient-corrected (P86) as well as their combinations are used to perform benchmark calculations on the water monomer and dimer. We use the augmented correlation-consistent polarized valence orbital basis set of double (aug-cc-pVDZ) and triple zeta quality (aug-cc-pVTZ) in order to compare the DFT with earlier MP2 results [J. Chem. Phys. 99, 8774 (1993); 100, 7523 (1994)]. Better overall agreement with the MP2 and experimental results for the water monomer and dimer is observed for the B–LYP and B–P86 functional combinations. The optimal structures, harmonic vibrational frequencies, and energetics of the clusters trimer through hexamer are computed at the B–LYP/aug-cc-pVDZ level of theory. This functional combination yields structures, energetics, and trends with cluster size that are in good agreement with the corresponding MP2 results. © 1995 American Institute of Physics.

Notes: We present the first nonempirically calculated spectroscopic constants for the recently observed [J. Chem. Phys. 105, 2167 (1996)] ground (X 2Σ+) and excited (A 2Π) states of Sr+Ar. Our best results yield De=694 cm−1, Re=3.662 Å, and ωe=38.7 cm−1 for the ground and De=1967 cm−1, Re=3.169 Å, and ωe=99.1 cm−1 for the excited state. The calculated De's are within the error bars of the experimentally determined one for the ground state favoring the low end and underestimate the corresponding one for the excited state by about 7%. The equilibrium separations (Re's) for the two states have not been experimentally determined, however our results accurately reproduce the estimated shift, ΔRe, between the two states. The interaction is mainly electrostatic for the ground state for which the contribution of dynamic electron correlation was found to be more important than for the excited state. © 1998 American Institute of Physics.

Notes: We present a parametrization of the interaction potential for Al3+ in water from first principles calculations. We have performed a critical study of the Al3+–water interaction using sequences of correlation consistent basis sets that approach the complete basis set limit and include core-valence correlation effects. We suggest as minimum theoretical requirements treatment of the electron correlation at the MP2 level of theory using a triple zeta quality basis set that accounts for the effect of core-valence correlation. The latter amounts for an increase of ∼5 kcal/mol (3%) to the stabilization energy, a shortening of 0.015 Å in the Al–O distance, and an increase of 22 cm−1 in the harmonic frequency of the Al–O vibration. This is the first time that core-valence effects were investigated for this system. The stabilization energy of the Al3+(H2O) cluster is 201 kcal/mol and the corresponding Al–O bond length is 1.719 Å at the MP2 level of theory with the cc-pwCVQZ basis set. This minimum is metastable with respect to the Al2++H2O+ asymptote since even the second ionization potential (IP) of Al is larger than the first IP of water. The hexa-aqua cluster Al3+(H2O)6 is, however, stable upon dissociation to Al3+(H2O)5+H2O by 64.8 kcal/mol, demonstrating the capacity of "effective" solvation in stabilizing the charge on the cation. The optimal structures of the n=5 and 6 clusters (having C2v and Th symmetries, respectively) and their harmonic vibrational frequencies are the first ones reported at the MP2 level with basis sets of this size. Core-valence correlation effects for the n=6 cluster are found to be of similar magnitude with those observed for the n=1 cluster. The stabilization energy of the n=6 cluster with respect to its fragments is 723.7 kcal/mol and the corresponding Al–O distance is 1.911 Å. These results were used in order to parametrize a pairwise-additive interaction potential for aluminum–water interaction that was grafted onto the Toukan–Rahman interaction potential for water. The potential model reproduces the ab initio results for Al3+(H2O)6 within 2.0 kcal/mol for the stabilization energy and 0.003 Å for R(Al–O) distance. Using this potential we estimated the enthalpy of solvation of Al3+ to be −1106±6 kcal/mol, therefore favoring the lower value of the experimentally obtained data (−1115 and −1140 kcal/mol, respectively). In addition, we calculate the first peak of the Al–O radial distribution function at 1.885 Å, in excellent agreement with x-ray diffraction studies that suggest a peak at 1.882±0.004 Å. We compute the first peak of the Al–H radial distribution function at 2.473 Å and the average angle between the plane of a water molecule and the Al–O vector at −28.27°. © 1997 American Institute of Physics.

Notes: We have computed potential energy functions for the ground states (X 3Σ−) of NF and NCl using a series of correlation consistent basis sets ranging from double to sextuple zeta quality and including core-valence correlation effects in conjunction with coupled-cluster single and double excitations with perturbative treatment of triple excitations [CCSD(T)] and large internally contracted multireference configuration interaction (icMRCI) wave functions. The best estimates for the dissociation energies (De's) are 76.6±1.3 kcal/mol for NF and 64.6±1.3 kcal/mol for NCl, respectively. Our results suggest that previous experimental estimates for the dissociation energy of NCl are in error by as much as 15 kcal/mol. The calculated spectroscopic constants for NF and NCl are in good agreement with the measured constants. © 1997 American Institute of Physics.